Osteoarthritis is a progressive disorder of the joints caused by loss of cartilage and is the most common form of arthritis. The loss of cartilage may result from a person's joint wearing down over time or through a sudden injury to the joint (e.g. through normal use, sudden injury/trauma, repetitive stress injuries, septic arthritis, repeated episodes of gout, poor posture, metabolic disorders, or secondary to obesity).
The loss of cartilage allows bones to rub together, causing pain, swelling, and loss of motion of the joint. As the cartilage is worn away, the bone forms spurs, areas of abnormal hardening, and fluid-filled pockets in the marrow known as subchondral cysts. As the disorder progresses, pain and inflammation results from deformation of the bones and fluid accumulation in the joints. As the injury worsens, the nervous system tunes to the pain signal. For instance osteoarthritis pain might be caused by degeneration of a person's knee, but the pain signal travels from the knee to the person's spinal cord and ultimately to the person's brain. During the course of osteoarthritis, the spinal cord and brain both become more sensitive to the pain signals, essentially becoming more efficient at processing the signals sent from the knee and amplifying the perception of the pain. Specifically, a chronic pain signal from an injured joint leads to peripheral pain receptors becoming sensitized to an injury. The chronic pain signal causes changes at the spinal cord that amplify electrical activity and the pain signal that is sent to the brain. The amplified pain signal sent to the brain results in increased activity in select regions of the brain, leading to central sensitization, i.e., increased pain perception, and amplified facilitory output from the brain. Osteoarthritis gradually worsens with time, however, treatment can relieve pain and allow a person to live a normal and active lifestyle.